Further investigation on normalized biomass water use efficiency.

The need to normalize biomass water use efficiency (B_WUE) for different climatic conditions and, mainly, for the evaporative demand of the atmosphere, has been largely established. Physiologically comparison of B_WUE can not disregard normalization especially in dealing with crop modelling. Normalization of B_WUE can take two routes: a) normalizing through the "transpiration gradient", nam ely vapour pressure deficit (VPD); b) normalizing through the "reference transpiration flux", namely ref erence evapotranspiration (ETo). The first route has shown to be extremely reliable when applied at leaf sc ale, while, when applied at canopy scale, it shows drawbacks particularly related to the approximation in to assuming the same value for both leaf (or canopy) and air temperature. The second route has demonstrated to be more effective than the first at canopy scale, especially if the Penman-Monteith equation, to compute ETo, is used, because both the "energy" and the "aerodynamic" components are combined in it. Thus, the normalization of B_WUE by ETo (B_WUE ), as compared to VPD, has proved ETo to be more reliable for application across a range of conditions differing in the evaporative demand and appropriate from well-irrigated to rainfed conditions. In this work, both examples from the scientific literature and data re-elaborated from previous experiments have shown the robust capacity of B_WUE, normalized by ETo, for C3 - C4 grouping. Moreover, within each group, a general consistency among B_WUE values of different crops has been found. In fact, many C3 crops position on values of about 13 g m2 , while C4 crops on values about 29 gm2 .The quite well-unified values allow to be extremely suitable for modelling purposes, aiming to predict crop growth through the "water driven growth-engine". The collection of some recent data of many different crops from the same experimental site in the Southern Italy (Policoro, MT, 40°17'N, 4°25'E) have confirmed the greater reliability of the "reference transpiration flux" as compared to the "transpiration gradient" approach for normalization. Furtherm ore, the range of B_WUE values for crops such as sugar beet, tomato, eggplant, pepper, rocket (C3) and ETo sweet maize (C 4) have been found similar to the figures reported above. Nevertheless, some winter crops (durum wheat, broad bean, celery, broccoli, broccoli raab) have a much lower biomass WUE value (about 7.2 g m2 ). This specific behaviour might be explained on the basis of a much lower thermal regimes during the winter season, interfering with a proper crop development, and of a higher root/shoot ratio. Further investigations are needed to pinpoint the underlining causes for such a low value of B_WUE in these ETo crops.